An electrocardiograph records electric potentials generated by the neuromuscular mechanism of the heart. A stimulus arising in the sino-auricular node of the heart sets up a tiny electric current called the excitation or depolarization wave. This wave spreads over the auricular wall and the auricles contract. When the excitation wave is at the junctional tissue between auricle and ventricle, it is delayed by the atrioventricular node at the beginning of the Bundle-of-His. The excitation wave conducts rapidly through the Bundle and then branches to the left and right ventricle.
The excitation wave then conducts slowly through the Purkinje fibers which terminate in the ventricular muscle, causing the ventricles to contract. A period of heart rest follows, after which a fresh impulse arises at the sino-aurircular node and the contraction- cycle once again repeats. By placing electrodes on the chest of a patient, a time varying voltage corresponding to the spreading excitation wave can be measured.
This excitation waveform, referred to as an electrocardiogram (ECG), is used to analyze the operation of the heart. The electrocardiogram is divided up into five time segments, known to those skilled in the art as, P, Q, R, S, and T which correspond to the different parts of the waveform. The "P" or Auricular Wave corresponds to the spread of the wave through the auricular musculature. At the point the wave spreads over the ventricular neural net of the Bundle-of-His, a rapid rise and fall of the wave potential, referred to as the "R" wave occurs. Before the "R" wave portion, a small dip called the "Q" wave occurs in the electrocardiogram while after the "R" wave a large drop in the electrocardiogram occurs called the "S" wave. The final portion of the electrocardiogram, the "T" wave, corresponds to the resting phase of the ventricle during which time an electrical repolarization of the ventricular muscle occurs. After the "T" wave the electrocardiogram reflects a horizontal line or isoelectric baseline indicating a period of heart rest prior to the next cycle of the excitation wave.
By analyzing deviations in the shape of the electrocardiogram, physicians can diagnose pathological conditions which relate to the heart and circulatory system. Typically the electrocardiogram is recorded by a paper chart recorder and examined for deviations in the shape of the various segments of the waveform.
Most chart recorders can accurately record waveforms occurring at frequencies up to 100 Hz. However, waveforms occurring at frequencies greater than 100 Hz are outside the bandwidth of the chart recorder and are distorted or lost when recorded since the changes in the waveform occur too rapidly for the pen or stylus of the chart recorder to follow. For example, conventional ECG chart recorders cannot display the small rapid variations in the waveforms, having frequencies typically in the range of 600 to 1000 Hz, referred to as notches and slurs (hereinafter referred to as "notches") which are common to the QRS complex of patients with clinical and preclinical heart disease.
In one method to overcome the limited bandwidth of an ECG chart recorder, ECG signals are routed for display to a cathode-ray tube. The display generated is then photographed to produce a permanent recording of the waveform. This method is used in research facilities as a research tool and is not readily accessible to cardiologists, internists, and general practitioners outside this environment.
U.S. Pat. No. 4,565,201 describes a signal averaging means which includes a microcomputer programmed to output information indicative of the ECG signal at a fraction of the speed of which it was input to the microcomputer. However, since the notches are small and difficult to detect, cardiologists, internists and other medical personnel cannot detect and analyze the notches with any degree of confidence.
One method to overcome the problem of visual detection and analysis of the notches is to amplify the signal. However since the amplitude of the notches is extremely small, (the ratio of the maximum amplitude of a ECG waveform to the maximum amplitude of a notch may be from 1:50 to 1:20) amplification of the entire waveform to increase the visibility of the notches would drive the signal off scale, saturating the display/recording device. Thus when the signal is amplified to display the notches, the entire ECG waveform cannot be recorded by the chart recorder. In addition, since the notches occur quite frequently near the minimum and maximum amplitudes of the waveform, simultaneous recording of all the notches at the desired amplification is extremely difficult. It has been found that it is preferable that not only the notches, but the entire waveform and the positions of the notches with respect to the waveform be displayed/recorded for proper analysis and diagnosis.
U.S. Pat. No. 3,809,071 describes a means to amplify and display low level signals without saturating the display/recording device. However in this method the larger amplitude signals which are amplified off the scale of the display/recording device are simply cut off at the point of saturation Thus only the small amplitude signals are accurately displayed/recorded.
It is an object of the present invention to provide a means for enhancing electrocardiogram signals such that the waveform may be accurately recorded on a conventional chart recorder.
It is an object of the present invention to provide a means for enhancing electrocardiogram signals in a manner that medical personnel can easily detect and analyze small amplitude, high frequency notches often found in patients with clinical heart disease.
It is an object of the present invention to provide an automatic electrocardiogram enhancement system which is simple for medical personnel to use.